1. Fiber Optic Transmission
SL/HL – Option F.3

2. Reflection/Refraction
A wave encounters a boundary between two mediums and cannot pass through
The angle of incidence is always equal to the angle of reflection
Refraction
When a wave passes through boundary into a new medium its speed changes
The wave will change directions based on the change in its speed

3. Refraction
If the wave speeds up it will bend away from the normal line
If the wave slows down it will bend towards the normal line
Snell’s Law
sinθi cr = sinθr ci

4. Critical Angle
When traveling into air from some medium, light will always speed up, thus increasing the angle
If it speeds up enough, the angle of refraction will be 90 degrees
This means that the refracted ray will travel along the edge of the boundary

5. Critical Angle
When light strikes the boundary at the critical angle or greater, the wave is totally reflected back into the first medium
Here n1 is the index of refraction of the medium the light starts in

6. Total Internal Reflection
Usually when a wave reaches a boundary between mediums it is partially reflected and partially refracted
When the critical angle is exceeded the entire wave is reflected back within the medium
The wave doesn’t lose any energy

7. Optical Fiber
Fiber optic cable is made of thin, clear glass or plastic
Once light enters the cable it is totally internally reflected until it reaches the far end
Actual optical fiber is step indexed
There is another layer between the core and the outside
This is so the fibers can be bundled together

8. Critical Angle
The critical angle is that angle of incidence for which the angle of refraction is 90 degrees.
n1 sinθc = n2 sin90
Θc = sin-1 (n1/n2)

9. Practice Problem
A ray of light in water (refractive index 1.33) is incident from water on a water-air boundary.
Calculate the critical angle of the water-air boundary.

10. Practice Problem
A ray is emitted at point P is internally reflected. What can you deduce about the refractive index of the liquid?
8cm
Air = 1.00
Liquid = ?
12cm

11. Dispersion Modal Material
Not all the waves that enter make it to the other end, only certain ones
The possible paths are called modes
Material
Because different frequencies have different refractive indices, they have different paths
These can both cause problems if the bits of data arrive out of order
More direct modes are faster
Laser light and single mode cable reduce these effects

12. Acceptance Angle
The maximum angle of incidence a ray entering a fibre resulting in total internal reflection is called the acceptance angle.
A = sin-1 √(n12 – n22)

13. Practice Problem
The refractive index of the core of an optical fibre is 1.50 and that of the cladding is Calculate the acceptance angle of the fibre.

14. Material Dispersion

15. Attenuation Attenuation is the opposite of amplification
As a signal travels through a cable it will slowly lose intensity as energy is lost
Attenuation is measured in decibels (dB)
The 10 at the beginning is to convert to decibels
Generally measured in dBkm-1

16. Amplifiers
Even with reshaping, signals still attenuate over the length of the cable
Amplifiers along the cable increase the signal strength to keep it going
The same equation for Attenuation applies to Amplifiers

17. Practice Problem
An amplifier amplifies an incoming signal of power 0.34mW to a signal of power 2.2mW. Calculate the power gain of the amplifier in decibels.

18. Practice Problem
A signal starting with 10mW of power is reduced by 19 decibels over 50km. Calculate the power lost per km.

19. Wavelength/Attenuation

20. Reshapers
Monomode fibers can eliminate modal dispersion and lasers cut down on material dispersion, but it is not completely eliminated
Over a long distance individual pulses can start to overlap each other
Every 40-60km is a reshaper which will detect and reshape the signal
Has its own laser which sends a ‘new’ signal

21. Noise
One advantage to using fiber optics is that it is not particularly susceptible to noise
Any noise that does occur is generally due to random light entering the end of the cable
The power ratio of noise to signal in fiber optics is generally in the range of or 10-18
Signal to
Noise (dB)

22. Practice Problem
The minimum SNR considered acceptable for a certain signal is 30dB. If the power of the noise is 2.0mW, calculate the least acceptable signal power.